Both tools have the same intention: to give you a clear idea of what a product is making on Amazon. However, the tools arrive at their conclusions using different methods:
SaleSpy tracks inventory levels for the listings you choose and then measures the difference in inventory between days to show how many units were sold in any given time period. This is a great tool to use when taking a deep dive into one particular listing to truly gauge how that product sells on Amazon.
On the other hand, The Chrome Extension pulls data for all the listings that show up on any Amazon page and then estimates the sales by plugging key metrics (BSR, Reviews, Page Traffic, etc.) into our state of the art algorithm. The Chrome Extension is best utilized when taking your first looks at a product category as a whole to determine its potential.
Both are extremely useful when researching for new products - and they can be utilized in a number of different ways - but pro-tip: they are most useful when used in conjunction with one another :)